This invention relates to an installation for pickling a metal band by passing the latter inside at least one oblong tank containing a pickling liquid.
We know that during the rolling of metal bands, it is necessary to subject the band to superficial pickling intended, primarily, for the removal of oxyde on the surface of the band. To this end, the band is made to pass continuously, along a longitudinal direction, through an installation comprising, generally, several oblong tanks, each filled with a bath of liquid acid, intended for dissolving the oxyde.
Such installations have been in service for a long time and subjected to various improvements. Usually, the band passes through several successive tanks filled with acid, over guiding and back-up means arranged at the inlet and the outlet of each tank, above the level of acid. The band makes thus a series of curved sections immersed successively into the different tanks and each of the shape of a chain resting, at its extremities, on the back-up means, arranged respectively at the inlet and the outlet of the tank. Such an installation is disclosed, for instance in U.S. Pat. No. 3,445,284.
The acid concentration of the pickling liquid is reduced together with the dissolution of the oxyde and the acid has therefore to be renewed.
To this end, according to a well known arrangement, disclosed for instance in U.S. Pat. No. 3,445,284, acid is made to flow from one tank to another, in the reverse direction of the passage of the band that passes successively through baths whose acid concentration increases gradually. The first tank, in the flow direction, contains therefore the liquid the most filled with iron oxide that overflows toward an external regeneration circuit enabling to send a purified and more concentrated liquid back to the last tank in the line.
Moreover, the band reaches the pickling installation in a relatively cold condition and heats up in contact with the liquid that, conversely, cools down.
In order to maintain the temperature of the liquid at the requested level, each tank can be associated with a heating circuit comprising a pump drawing a portion of the liquid contained in the tank in order to send it back into the said tank, after being reheated in an heat exchanger.
Besides, the duration of the treatment depends on the flow speed of the band and on the depth of immersion in each tank, which depends on the strain applied to the band. In known installations of the type disclosed in U.S. Pat. No. 3,445,284, relatively deep tanks are used in order to limit the strain applied to the band. The consequence is the production of a large consumption of acid and an emission of very corrosive vapours which, even if they are caught and exhausted, may damage the various devices placed in the vicinity of the pickling installation.
To avoid these shortcomings and to reduce the quantity of acid necessary, it has been suggested to pass the band horizontally inside at least one oblong chamber of small height, closed by a lid and provided with lateral injectors. Thus, the band is not immersed in successive tanks any longer, but is subjected directly to acid jets. This chamber is placed in a tank that collects the acid that might escape through the openings of the band located at the inlet and the outlet of the chamber.
In such an arrangement, disclosed for instance in U.S. Pat. No. 2,418,386, the quantity of acid involved is reduced since the band need not be immersed into a bath, whereas the acid is injected directly over the band and renewed permanently. Besides, the emissions of acid are smaller than in the conventional tanks.
However, in such an installation, the band must be maintained rectilinear inside the treatment chamber and it is therefore supported by a series of aligned studs, provided on the floor of the chamber and delineating a horizontal plane for the band to run.
When running, the band slides over back-up studs. The pressure of application can be reduced by stretching the band, but the applicable tensile strain is necessarily limited, in particular in the case of very thin and relatively fragile bands. Frictions that may damage the surface of the band cannot then be avoided totally.
This invention relates to a new type of installation enabling to overcome all these shortcomings, while keeping the advantages of the pickling installations used until now.
Generally speaking, the pickling installation according to the invention is of the type comprising at least one oblong tank containing a liquid bath through which the band runs while making, inside the tank an upward concave curve with two extremities each located at an upper level determined by means for backing up the band, placed at both ends of the tank and a central portion lying at a lower level, immersed in the bath. This curve, in the form of a chain, has a deflection corresponding to the level difference between both ends and the central portion and whose value depends on the tensile strain applied to the band.
According to the invention, each tank is provided with at least one row of injection nozzles distributed along at least one of the longitudinal walls of the tank and linked to a pressurised supply circuit filled with pickling liquid in order to form at least one series of liquid jets penetrating inside the bath and the said nozzles are offset longitudinally and placed at different levels which, for each nozzle, depend on the longitudinal distance of the said nozzle with respect to an extremity of the tank and correspond substantially to the level of a portion of the band located at the same distance from the said extremity, so that the liquid jet injected into the bath by each nozzle is directed to the portion of the band passing in front of the nozzle in question.
Generally speaking, the arrangement of the nozzles is determined in relation to the shape taken by the band in the normal running conditions and, while the band is running, the tensile strain applied to the band is adjusted so that the curve formed by the band is maintained in permanence at each nozzle.
Preferably, the injection nozzles are distributed into two rows arranged respectively along both longitudinal walls of the tank and offset longitudinally with respect to one another so that the jet of a nozzle from a longitudinal wall passes between the jets emitted by two nozzles of the other wall.
Besides, the nozzles are advantageously oriented so that the jet of injected liquid is centred on a direction forming a non-right angle with the longitudinal running direction of the band, preferably against the running direction of the said band.
According to another advantageous feature, the bottom of the tank has an upward concave shape that is substantially parallel to the shape of the curve followed by the band under normal running conditions of the said band, whereas the tensile strain on the band is adjusted so that the curve formed by the band remains permanently substantially parallel to the bottom and spaced from the said bottom by a safety distance that is sufficient to avoid any contact between the band and the bottom while the band is running.
Besides, the pickling liquid supply circuit comprises at least one pump leading to a manifold to which are connected feeding ducts for each injection nozzle. Preferably, this pump is of the centrifugal type and is driven into rotation by a motor whose speed is adjusted in order to adapt the flowrate and the pressure of the liquid injected to the pickling conditions to be ensured for the band.
According to another particularly advantageous feature, each tank is provided with at least one exhaust orifice provided in at least one of the longitudinal walls of the tank for maintaining, thanks to an overflow, the surface of the pickling bath at substantially constant level. It is, moreover, particularly advantageous to use at least two exhaust orifices placed at two different levels with respect to the upper level of the back-up means and each associated with a removable closing means, for maintaining selectively the level of the surface of the bath at two levels at least, by opening one of the orifices and closing the other, whereas the level of the orifice kept open is determined in relation to the running speed of the band in order to suit the immersion length in each tank to the said running speed.
Preferably, each exhaust orifice leads to an exhaust caisson linked to a storage tank to which is connected a pressurised supply circuit of the injection nozzles by recycling the pickling liquid overflowing from the tank, whereas the said circuit comprises a means for reheating the recycled liquid.
Advantageously, the installation comprises a plurality of tanks, each associated with a storage tank and through which the band circulates in succession, all these tanks are linked to one another by ducts in order to enable the liquid to circulate from one tank to another in reverse direction of the running direction of the band, while increasing, from one tank to the next, the concentration in pickling liquid acid, whereas the tank associated with the uppermost tank in the running direction of the band is connected to purification and regeneration means of the liquid and whereas the regenerated liquid is sent back to the lowermost tank in the running direction of the band.